Carbon fibers, production process therefor and electrode for batteries

ABSTRACT

Fine carbon fibers having a fiber diameter of 1 μm or less, an interlayer distance d 002  between carbon layers as determined by an X-ray diffraction method of 0.335 to 0.342 nm or less and satisfying d 002 &lt;0.3448-0.0028 (log φ) where φ stands for the diameter of the carbon fibers, and a thickness Lc of the crystal in the C axis direction of 40 nm or less. Fine carbon fibers containing boron in crystals of the fiber. The fine carbon fibers can be produced by using vapor-grown fine carbon fibers as a raw material, adding a boron or a boron compound to the material; pressing the mixture to regulate the bulk density to preferably 0.05 g/cm 3  or more; and heating at a temperature of 2000° C. or higher.

REFERENCE TO THE RELATED APPLICATION

This is a divisional of application Ser. No. 09/937,709 filed Sep. 21,2001 now U.S. Pat. No. 6,489,026, which is a National Stage Entry ofPCT/JP00/01835 filed Mar. 24, 2000, which claims benefit of ProvisionalApplication No. 60/145,266 filed Jul. 26, 1999; the above noted priorapplications are all hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to fine carbon fibers (including coil-likecarbon fibers, vapor grown carbon fibers, whisker carbon fibers,expanded carbon fibers and other fibrous carbons) and a process forproducing the same, which carbon fibers are used as a filler added toany of a variety of materials such as metals, resins, and ceramics so asto improve electric and thermal conductivity thereof, as anelectron-emitting element for an FED (field-emission display), and as afiller such as a property-improving material for a variety of batteries.The invention also relates to an electrode for batteries; i.e., apositive and a negative electrode for any of a variety of batteries suchas a dry battery, a Pb secondary battery, a capacitor, and an Li ionsecondary battery, the electrodes being incorporated with fine carbonfibers so as to improve the charge-discharge capacity and the mechanicalstrength of an electrode plate.

BACKGROUND OF THE INVENTION

Fine carbon fibers, as referred to in the present invention, aretypically produced through a vapor phase method comprising thermaldecomposition of hydrocarbon (e.g., disclosed in Japanese PatentApplication Laid-Open (kokai) Nos. 7-150419, 5-321039, 60-215816, and61-70014 and Japanese Examined Patent Publication (kokoku) Nos. 5-36521and 3-61768). The thus-produced carbon fibers typically have a diameterof approximately 0.01-5 μm. If the diameter is 0.01 μm or more, the finecarbon fibers include carbon nano-tubes and carbon nano-fibers having aconcentric or tree-annual-ring structure similar to the carbon fibersgrown by the vapor phase method.

There has been proposed use of fine carbon fibers as a filler formetals, resins, ceramics, etc. Of these, fine carbon fibers areparticularly proposed as a filler for batteries, because the developmentof portable apparatuses such as small-sized cellular phones, videocameras, and notebook-type personal computers has been remarkable, andthe demand for a small-sized secondary battery including an Li ionsecondary battery (Li battery) serving as a power source therefor hasincreased drastically.

Examples of typical carbonaceous materials for a negative electrode ofan Li battery include hard carbon pitch, meso-phase carbon microbeads(MCMB), meso-phase pitch carbon fiber (MPCF), artificial graphite, coke,and naturally occurring graphite. Furthermore, the addition of carbonfibers produced from pitch or vapor-grown carbon fibers to thesenegative electrode materials has also been proposed, while carbonaceousmaterials such as graphite micropowder and carbon black are incorporatedinto a positive electrode as electric conductivity-providing agents.

In a negative electrode of an Li battery, intercalation anddeintercalation of lithium ions occur during charge and dischargeprocesses. Graphite, having a layered structure, easily undergoesintercalation wherein a reactant (e.g., Li) is inserted into aninterlayer void to thereby expand the void (intercalation). The reactionproduct containing a compound inserted in the interlayer void is calleda graphite intercalation compound. The intercalation compound easilyreleases the intercalated reactant (deintercalation), to therebytransform itself into graphite; i.e., the original material. Fine carbonfibers, which is a material having excellent electric and thermalconductivity and an intercalating property, do not lower the capacity ofthe battery, and thus addition thereof is of interest as an additive ina negative electrode material.

The enhancing of the intercalating property of the negative electrode isessential for enhancing the capacity of an Li battery. In general, inorder to enhance the intercalating property, the degree ofgraphitization; i.e., crystallinity, of carbonaceous materials includingfine carbon fibers must be enhanced.

A negative electrode of a lead secondary battery per se comprises acompound having poor electric conductivity, and carbonaceous materialssuch as carbon black, graphite microparticles and carbon fiber mayoptionally be added thereto to enhance the conductivity of the negativeelectrode. Such carbonaceous materials desirably have high electricconductivity and crystallinity. In order to enhance the crystallinity ofsuch carbonaceous materials, the materials are typically treated at hightemperature in order to effect graphitization.

Meanwhile, fine carbon fibers having a small average fiber diameter,particularly 1 μm or less, have a low bulk density and provide aninsufficient filling density. Thus, when fine carbon fibers are added,to an electrode, in a large amount, the density of the electrodedecreases. Therefore, fine carbon fibers are typically added in anamount of 20 mass % or less, preferably 10 mass % or less. From thispoint of view, fine carbon fibers are considered not to impart aremarkable effect commensurate with addition thereof even when thecrystallinity of the fibers is enhanced. Therefore, no investigationother than treatment at high temperature has been carried out so as toenhance the crystallinity of fine carbon fibers. Thus, conventionallyused fine carbon fibers have insufficient crystallinity, and thedistance between two crystal layers represented by d₀₀₂ is larger than0.3385 nm.

To meet with demand for increased capacity, electrode materials with alower electric resistance are sought to give a large electric currentcharge-discharge capacity.

Addition of various conductivity-imparting materials are studied tolower the resistance of electrodes, in which it is known that a fillerof a fibrous material particularly of vapor grown carbon fibers iseffective. The reasons thereof include:

1) fine fibrous materials have an aspect ratio of 100 or more allowing along conducting path;

2) vapor-grown carbon fibers have a high crystallinity to give a highconductivity; and

3) vapor-grown carbon fibers have a charge-discharge capacity to therebyprevent reducing the capacity of a Li battery when added.

However, commercially available fine fibrous materials having a diameterof 1 μm or less have an upper limit of the conductivity of 0.01 Ω·cm asthe powder resistance when measured at a density of 0.8 g/cm³ and thereare no available fine fibrous materials having a conductivity higherthan this.

Recently, the crystallinity of a negative electrode material has beenenhanced to thereby improve the charge-discharge capacity of a battery.This trend forces an additive other than a negative electrode materialto have a high discharge capacity. Therefore, the crystallinity ofcarbonaceous material serving as an additive for a negative electrodemust be enhanced.

Considering the above requisite to enhance the crystallinity of finecarbon fibers, the inventors have been investigated heat treatment up to3200° C. to enhance the crystallinity.

However, even when fine carbon fibers (vapor-grown carbon fibers) havingdiameters of the order of about 0.15 μm are heated to 3000° C. orhigher, the lattice constant of the interlayer distance d₀₀₂ cannot bereduced to less than 0.3385 nm.

Simultaneously, the conductivity had an upper limit of 0.01 Ω·cm as thepowder resistance when measured at a density of 0.8 g/cm³. Accordingly,fine carbon fibers having a higher crystallinity and a lower resistanceare sought.

The supposed reason for the above failure is that vapor-grown carbonfibers have a considerably small fiber diameter and a unique structurewherein the core of the concentrically grown crystalline fiber comprisesa hollow or amorphous portion. In addition, when the fiber diameter isas small as 1 μm or less, maintenance of a roll-shape structure of acarbon hexagonal network plane becomes more difficult with decreasingdistance between the structure and the center of the fiber, to therebymake crystallization difficult. Therefore, the value of d₀₀₂ depends onthe diameter of the fiber. For example, the limitations on theinterlayer distance d₀₀₂ were 0.3385 nm for the fiber with a diameter ofabout 0.15 μm, 0.3400 nm for the fiber with a diameter of about 0.05 μm,0.3415 nm for the fiber with a diameter of about 0.02 μm, and 0.3420 nmfor the fiber with a diameter of about 0.01 μm or less. Therefore, thelimitation of the interlayer space d₀₀₂ is 0.3385 nm and, even when thecarbon fibers are heated to 3000° C. or higher, d₀₀₂ cannot be reducedto less than 0.3385 nm.

Accordingly, in order to enhance crystallinity and to reduce d₀₀₂ to0.3385 nm or less, there must be developed a method for enhancingcrystallinity other than heat treatment.

The object of the present invention is to develop fine carbon fibershaving a high crystallinity that has not been produced through aconventional method or having a high crystallinity, and to provide abattery electrode having a higher performance by containing thedeveloped carbon fibers as a filler.

SUMMARY OF THE INVENTION

Firstly, the present inventors have investigated a graphitizationcatalyst (Called also a graphitization aid or additive and hereinaftersimply called a “graphitization catalyst” or “catalyst”) so as to attainthe above-described objects.

Before now, no investigations have been conducted on controlling thecharacteristics of fine carbon fibers having a fiber diameter of 1 μm orless by use of a graphitization catalyst. Further, the degree ofenhancement in the crystallinity and characteristics of fine carbonfibers having such a particular crystal structure by use of agraphitization catalyst has not been elucidated.

Secondly, the present inventors have investigated treatment of finecarbon fibers by use of a catalyst.

The present invention has been accomplished based on the above study.The present invention provides the following.

(1) Fine carbon fibers having a diameter of 1 μm or less; an interlayerdistance d₀₀₂ between carbon layers as determined by an X-raydiffraction method in a range of 0.335 nm to 0.342 nm, further 0.3354 nmto 0.3420 nm, and satisfying d₀₀₂<0.3448-0.0028 (log φ), preferablyd₀₀₂<0.3444-0.0028 (log φ), more preferably d₀₀₂<0.3441-0.0028 (log φ),wherein 4 stands for the diameter of the carbon fibers and the units ofd₀₀₂ and φ are nm; and a thickness Lc of the crystal in the C axisdirection of 40 nm or less.

(2) Fine carbon fibers according to (1), characterized in that the Rvalue of Raman spectrum is 0.5 or more and the peak width of half heightof a peak of the spectrum at 1580 cm⁻¹ is 20-40 cm⁻¹.

(3) Fine carbon fibers having a diameter of 1 μm or less and containingboron in the crystals of the fibers.

(4) Fine carbon fibers according to (1) or (2), which contain boron inthe crystals of the carbon fibers.

(5) Fine carbon fibers according to (3) or (4), which contain boron inan amount of 0.1-3 mass %.

(6) Fine carbon fibers according to any one of (1) to (5), which have afiber diameter of 0.01-1 μm and an aspect ratio of 10 or more.

(7) Fine carbon fibers according to (6), wherein the resistance as thepowder when pressed to a density of 0.8 g/cm³ is 0.01 Ω·cm or less inthe direction vertical to the pressing direction.

(8) Fine carbon fibers according to any one of (1) to (7), which arevapor-grown carbon fibers.

(9) A process for producing fine carbon fibers which comprises addingfine carbon fibers having a diameter of 1 μm or less with boron or aboron compound and heating the fine carbon fibers to 2000° C. or higher.

(10) A process for producing fine carbon fibers which comprises addingfine carbon fibers having a diameter of 1 μm or less with boron or aboron compound;

-   -   regulating bulk density of the fine carbon fiber to 0.05 g/cm³        or more; and

heating the fine carbon fiber to 2000° C. or higher while said densityis maintained.

(11) A process for producing fine carbon fibers according to (9) or (10)wherein the amount of the added boron or boron compound is 0.1 to 10mass %.

(12) A process for producing fine carbon fibers according to any one of(9) or (11) wherein the fine carbon fibers to which the boron or boroncompound is added have a diameter of 0.01 to 1 μm and an aspect ratio of10 or more.

(13) A process for producing fine carbon fibers according to any one of(9) or (12) wherein the fine carbon fibers to which the boron or boroncompound is added are vapor-grown carbon fibers.

(14) A process for producing fine carbon fibers according to (13)wherein the fine carbon fibers, prior to said heat treatment, to whichthe boron or boron compound is to be added are fired products ofvapor-grown carbon fibers fired after growth.

(15) A process for producing fine carbon fibers according to (13)wherein the fine carbon fibers, prior to said heat treatment, to whichthe boron or boron compound is to be added are unfired products ofvapor-grown carbon fibers not fired after the growth.

(16) A battery electrode containing fine carbon fibers as recited in anyone of (1) to (8).

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between the fiber diameter offine carbon fibers and the interlayer distance of the graphite crystal;and

FIG. 2 is a cross section of an apparatus for measuring the resistanceas the powder of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

The fine carbon fibers of the present invention have excellentcrystallinity, and the interlayer distance d₀₀₂ between carbon crystallayers as obtained through an X-ray diffraction method is 0.335-0.342 nmand the thickness Lc in the C axis direction of crystal is 40 nm orless, preferably 32 nm or less.

FIG. 1 is a graph showing the function of the interlayer distance d₀₀₂as in relation to the fiber diameter for fine carbon fibers which havenot been treated with boron and which have been treated with boron,which measurement was conducted by the inventors. The fine carbon fiberswhich have not been treated with boron have an interlayer distance d₀₀₂greater than those represented by d₀₀₂=0.3448-0.0028 (log φ) where φstands for the diameter of the carbon fiber, but the fine carbon fiberswhich have been treated with boron have the interlayer distance d₀₀₂substantially smaller than those represented by the same formula.Therefore, the fine carbon fibers can be defined as having an interlayerdistance d₀₀₂ as represented by d₀₀₂<0.3448-0.0028 (log φ), preferablyd₀₀₂<0.3444-0.0028 (log φ), more preferably d₀₀₂<0.3441-0.0028 (log φ),where φ stands to the diameter of the carbon fibers.

In accordance with the study by the present inventors, the interlayerdistance d₀₀₂ of carbon crystal could not be made smaller thand₀₀₂=0.3448-0.0028 (log φ). Here, even if such small d₀₀₂ can beattained without boron treatment, such would be able to be attained onlyby conducting a special treatment or by attaining severe processconditions or the control thereof, the significance of the presentinvention, where fine carbon fibers having a small interlayer distanced₀₀₂ in the range of the present invention can be easily obtained bytreating with boron, is not lost.

The fine carbon fibers of the present invention can be carbon fiberscontaining boron. The boron in the carbon fibers is present in thecrystal of carbon (graphite), between the crystal layers, or in thecrystal grain boundaries, or as impurity.

The fine carbon fibers of the present invention containing boron incrystals of the fibers are novel and therefore are not limited by theranges of d₀₀₂ and Lc as above mentioned. The fibers preferably containboron, and the d₀₀₂ and the Lc preferably fall within theabove-described ranges.

Fine carbon fibers containing boron, and in which d₀₀₂ and Lc fallwithin the above-described ranges, can possess an R value of 0.5 ormore, as obtained from a Raman spectrum (R=ID/IG, ratio of theabsorption intensity ID at 1360 cm⁻¹ to the absorption intensity IG at1580 cm⁻¹) and a narrow peak width at half height of peak at 1580 cm⁻¹of the spectrum of 20 to 40 cm⁻¹.

The fine carbon fibers preferably have a fiber diameter of 0.01-1 μm,and, in view of the development of a function as fibers, an aspect ratioof 10 or more, more preferably 50 or more.

A fiber diameter of less than 0.01 μm induces poor mechanical strengthwhen such fibers are used as a battery electrode or a filler for resin,a function as a fiber is easily lost; e.g., the fiber is easily cut.Meanwhile, for a given amount of addition as a filler (mass %), anincrease in fiber diameter has an effect of reducing the number offibers to be added. In this case, characteristics of fibers as a fillerare insufficiently exhibited. When the fiber diameter is great, suchcarbon fibers do not easily enter into interparticle spaces of graphiteparticles contained as an electrode material in a battery negativeelectrode comprising carbon. When carbon fibers have a fiber diameter inexcess of 1 μm, the productivity of the fibers per se decreasesconsiderably, to thereby disadvantageously induce an increase inproduction cost. Thus, the fiber diameter is preferably 1 μm or less,more preferably 0.5 μm or less.

No particular limitation is imposed on the length of the carbon fibers,and the lower limit thereof is preferably determined from the lowerlimit of the aspect ratio (fiber length/fiber diameter). When the fiberlength is excessively long, dispersibility when the fibers are used as afiller disadvantageously decreases due to entanglement of fibers. Thus,the upper limit of the fiber length is preferably 400 μm, morepreferably 100 μm. For example, when the aspect ratio is 50 or more, afiber diameter of 0.01 μm requires a fiber length of 0.5 μm or more,whereas a fiber diameter of 0.1 μm requires a fiber length of 5 μm ormore. Also, the upper limit of the fiber length is preferably 400 μm,more preferably 100 μm.

The fine carbon fibers of the present invention having highcrystallinity can be produced by heat treating fine carbon fibers in thepresence of a boron compound. Although it is not intended to be bound bythe theory, it is considered that, by such heat treatment, boron isincorporated into carbon fibers and the catalytic action of theincorporated boron gives the fine carbon fibers of the present inventionhigh crystallinity

The amount of boron in the fine carbon fibers effective in inducing ahigh degree of crystallization is generally in a range of 0.1-3 mass %,preferably 0.2-3 mass %. Boron is diffused through, for example, heatingat high temperature, after completion of crystallization to a highdegree. However, the boron content may become lower than the amount ofboron added, so long as the fiber contains boron during crystallization.

The process for producing fine carbon fibers according to the presentinvention will next be described.

Carbon Fibers Serving as a Starting Material:

In the process according to the present invention, vapor-grown finecarbon fibers obtained through thermal decomposition of an organiccompound such as benzene may be used as a starting material. Forexample, the starting material can be produced through methods disclosedin Japanese Patent Application Laid-Open (kokai) Nos. 7-150419,5-321039, 60-215816, and 61-70014 and Japanese Examined PatentPublication (kokoku) Nos. 5-36521 and 3-61768. As long as the diameteris 0.01 μm or more, fine carbon fibers as carbon nano-tubes or carbonnano-fibers having a tree-annual-ring structure can be also used.Therefore, carbon nano-tubes, carbon nano-fibers or the like, having amultiple structure produced by arc discharge method, laser method, etc.can be also used.

The process for producing fine carbon fibers in a vapor growing methodwill be briefly described. A transition metal or a compound thereof, forexample, a super fine powder of a metal such as iron, nickel or cobaltor super fine particles based on ferrocene is used as a seed; the seedsuper fine powder or super fine particles are formed on a substrate; astarting carbon material and optionally a carrier gas such as hydrogenare supplied thereto; the starting carbon material is decomposed at ahigh temperature; and fine fibers having a fiber diameter of 0.01 to 1μm or more are grown with the super fine powder or super fine particlesfunctioning as seeds. The method for forming a seed includes, on asubstrate (the wall of a furnace may be used as a substrate), applyingand drying a seed particle dispersion or a seed source solution,spraying ferrocene or the like, and forming fine particles of iron or acompound thereof by using ferrocene in a flowable state, and the seedmay be formed on a surface of a substrate or may be formed in a flowingbed.

Moreover, because, on the surface of fine carbon fibers as grown by avapor growing method as above, a large amount of tars and low boilingpoint components are simultaneously grown and fixed thereto and highlyactive iron fine particles are also present, the carbon fibers may beheated to treat the above substances.

However, the present inventors have found that the crystallinity of thefine carbon fibers increases insufficiently through heating. Theinventors have investigated a catalyst (aid or additive) for attaining ahigh degree of crystallization. Examples of possible catalysts includeB, Al, Be, and Si. Of these, boron (B) is particularly effective.Conventionally, there have been studied enhancement of crystallinity intypical carbonaceous materials through mixing with boron and heating(e.g., “TANSO” 1996, No. 172, p89-94; and Japanese Patent ApplicationLaid-Open (kokai) Nos. 3-245458, 5-251080, 5-266880, 7-73898, 8-31422,8-306359, 9-63584, and 9-63585).

However, no investigation has been conducted for improving acharacteristic of vapor-grown fine carbon fibers having a fiber diameterof 1 μm or less through incorporation of boron. The reason is that sincesuch fine carbon fibers have a unique structure the catalytic effectprovided by boron in typical carbonaceous material has been consideredunobtainable.

That is, vapor-grown carbon fibers have a cylindrical structure having aplurality of concentric layers, and a crystal structure is grownconcentrically in a cross-section. The length of the fiber varies inaccordance with production conditions. Since fibers having a small fiberdiameter such as approximately 0.01-1 μm exists in the form of a singlefilaments or branched filaments, definite determination of the length isdifficult. Through measurement of linear portions under a scanningelectron microscope, most of the fiber filaments are found to have afiber diameter of 5 μm or more. Since the fibers exist in the form ofsingle long filaments or branched filaments, both long filaments andshort filaments having a length of approximately 5 μm easily form aflock having a dimension of 10 μm or more, occasionally 100 μm or more.Thus, the fiber mass has a bulk density as small as 0.05 g/cm³ or less,typically 0.01 g/cm³ or less. In addition, a flock-like structure isformed.

Vapor grown carbon fibers having a unique structure different from usualcarbon fibers are difficult to contact with a graphitization catalyst,to thereby possibly fail to attain a homogeneous introduction of boron.

In fine carbon fibers, as the diameter of the fibers becomes finer,particularly near the center, a carbon crystal layer tends to be bent,where it was thought unlikely that the fine carbon fibers are maintainedwhen the crystallinity is enhanced by making the interlayer distance ofthe carbon crystal smaller. In other words, it was thought unlikely thatthe crystallinity of fine carbon fibers can be enhanced by boron.

However, the inventors, as a result of intensive efforts, have attainedcarbon fibers with a high cryatallinity by using boron as a catalyst(aid) in vapor grown fine carbon fibers.

In accordance with the present invention, in order to carry out dopingwith boron, there is used starting carbon fibers heat treated at lowtemperature, preferably at 1500° C. or lower, or more preferably asgrown and not heat treated, which are not completely crystallized andare easily doped with boron. Even when starting fibers are not heattreated, the fibers are finally heated to the graphitization temperatureduring treatment with a catalyst comprising boron (boron-introducingtreatment). Thus, starting fibers having a low crystallization degreecan be used satisfactorily. Although carbon fibers graphitized at 2000°C. or higher, preferably 2300° C. or higher, may also be used, suchgraphitization is not necessary. Simultaneous graphitization andcrystallization of carbon fibers not heat treated by use of a catalystis preferred in view of energy conservation.

Fine carbon fibers serving as a starting material may be disintegratedor crushed in advance, to thereby facilitate handling in a subsequentstep. The disintegration or crushing is sufficient if it is carried outto a degree such that mixing with boron or a boron compound is possible.In other words, before doping with boron the starting fibers are notnecessarily processed so as to have a length suitable for a filler,since, after doping with boron, the starting fibers are finallysubjected to treatment for preparing a filler, such as disintegration,crushing, or classification. Vapor-grown carbon fibers having a fiberdiameter of approximately 0.01-1 μm and a length of approximately0.5-400 μm may be used as such. The fibers may be flocculated. Thestarting fibers may also be subjected to heat treatment. In this case,the treatment is preferably carried out at 1500° C. or lower.

Boron or a Boron Compound:

Boron or a boron compound which is used in boron-introducing treatmentsuitably has the following properties, although it is not particularlylimited. Since the treatment is carried out at 2000° C. or higher, thereis used a substance which is not vaporized through thermal decompositionbefore the temperature is raised to at least 2000° C. Examples of thesubstance include elemental boron, B₂O₃, H₃BO₄, B₄C, BN and other boroncompounds.

Typically, carbon can be doped with boron in an amount of 3 mass % orless. Thus, boron or a boron compound in a starting mixture may be in anamount of 10 mass % or less as reduced to boron atoms based on carbonatoms, in consideration of the doping ratio. When the amount isexcessive, the cost for the treatment increases, and the additive iseasily sintered or solidified, or covers the surface of the fibers,whereby characteristics of a filler may fall, e.g., electric resistanceincreases.

The fine carbon fibers (fiber diameter of 1 μm or less), having a3-dimensional structure, are easily flocculated and have considerablylow bulk density and high porosity. In addition, since boron is added inan amount as low as 10 mass % or less, preferably 5 mass % or less,attaining homogeneous contact between the fiber and boron species isdifficult through simple mixing of the two components.

In order to effectively introduce boron, fibers and boron or a boroncompound are sufficiently mixed, to thereby make contact ashomogeneously as possible. In order to attain homogenous contact, boronor a boron compound employed must have as small a particle size aspossible. When the particles are large, a domain containing highconcentration of boron species is locally generated, to thereby possiblyinduce solidification. Specifically, the average particle size is 100 μmor less, preferably 50 μm or less, more preferably 20 μm or less.

A compound such as boric acid may be added in the form of an aqueoussolution, and water may be vaporized in advance, or water may bevaporized during a heating step. If the aqueous solution is mixedhomogeneously, a boron compound can be homogeneously deposited on thesurface of the fibers after removal of water through vaporization.

As described above, the vapor-grown fine carbon fibers have a small bulkdensity, and the as-produced mass has a bulk density of approximately0.01 g/cm³ or less. Even a product obtained through heat-treating ordisintegrate-crush-classify treatment of the mass has a bulk density ofapproximately 0.02-0.08 g/cm³. Such a low bulk density is,attributed tothe fine carbon fibers being easily flocculated. Since the vapor-grownfine carbon fibers have high porosity, a considerably large-scalefurnace is required for heat treatment, to thereby disadvantageouslyincrease facility cost and lower productivity. Accordingly, a method foreffectively introducing boron into a carbonaceous material other than acustomary one must be developed.

In order to effectively carry out introduction of boron, contact betweencarbon and boron must be sufficiently maintained. The two componentsmust be homogeneously mixed, to thereby attain sufficient contacttherebetween. Furthermore, there must be prevented separation of the twocomponents during heat treatment, which tends to cause unevenness inconcentration.

Although the carbon fibers and boron or a boron compound may be mixedhomogeneously and heated, the mixture is preferably treated so as toattain high density and is heated while the density is maintained(solidified) as long as possible. In a preferred mode of the presentinvention, two starting materials are mixed, and the mixture iscompacted with pressure, to thereby solidify the mixture and increasedensity prior to heat treatment.

No particular limitation is imposed on the method for mixing carbonfibers and boron or a boron compound, so long as homogeneity ismaintained. Although a variety of commercially available mixers may beused, a Henschel mixer having a chopper for disintegrating flocks ispreferred as fine carbon fibers are easily flocculated. The startingfibers which are used may be as-produced fibers or fibers treated at1500° C. or lower. Of these, as-produced fibers are preferred, in viewof production cost.

There may be employed any of a variety of methods for solidifying amixture of carbon fibers and boron or a boron compound so as to increasedensity and prevent separation of the two components. For example, theremay be used shaping, granulating, or a method in which a mixture iscompacted in a crucible into a certain shape. In the case in which theshaping method is used, the shaped mixture may assume any of columnar,plate-like, or rectangular parallelepiped form.

The above mixture which is solidified to have a high density has a bulkdensity of 0.05 g/cm³ or more, preferably 0.06 g/cm³ or more.

When the pressure for compacting the mixture is relieved aftercompletion of shaping, the shaped mixture may be expanded, to therebylower the bulk density. In consideration of such a case, the bulkdensity obtained during compacting is controlled such that thesolidified mixture has a density of 0.05 g/cm³ or more after pressurerelief. When the carbon fibers are placed in a container, the fibers maybe compacted for enhancing the treatment efficiency by use of a pressingplate or the like to thereby attain a bulk density of 0.05 g/cm³ ormore. The fibers may also be heated while it is being compacted.

The thus-treated fibers; i.e., a mixture of the fibers and boron or aboron compound and having an enhanced bulk density, is then heated.

In order to introduce boron into a crystal of carbon, the mixture mustbe heated at 2000° C. or higher, preferably 2300° C. or higher. When thetemperature is less than 2000° C., reactivity between carbon and boronis poor, to thereby prevent introduction of boron. The temperature ispreferably 2300° C. or higher so as to further accelerate introductionof boron, enhance the crystallinity of carbon, and, particularly,regulate d₀₀₂ to 0.3385 nm or less for a fiber having a diameter ofabout 0.15 μm. Although there is no upper limit of the temperature ofheat treatment, the limit is approximately 3200° C. in accordance withrestrictions imposed by an apparatus.

No particular limitation is imposed on the furnace used in heattreatment so long as the furnace can maintain a target temperature of2000° C. or higher, preferably 2300° C. or higher. Examples of typicallyemployed furnaces include an Acheson furnace, an electric resistancefurnace, and a high-frequency furnace. Alternatively, a powder of amixture or a shaped powder may be heated through direct application ofelectric current.

The atmosphere for heat treatment is a non-oxidizing atmosphere,preferably an inert gas such as argon. In view of productivity, theheating time is preferably as short as possible. Particularly, heatingfor a long period of time promotes sintering, to thereby lower theyield. Therefore, after the temperature of a core portion of the shapedmixture has reached the target temperature, further maintenance at thetemperature for one hour or less is sufficient.

Through the heat treatment, the carbon fibers of the present inventionfirst attain a d₀₀₂ of 0.3420 nm or less and enhanced crystallinity.However, Lc (a thickness of the layers in the direction of the C axis ofa carbon crystal as determined by the X-ray diffraction method) stillremains 40 nm or less, more preferably 32 nm or less, which is equal tothat of similarly heat-treated B-free carbon fibers. Generally, ingraphitizing carbonaceous material, Lc typically increases as d₀₀₂decreases. In the carbon fibers having a diameter of about 0.2 μm ofpresent invention, Lc does not increase, but remains at 40 nm or less,which is equal to that of similarly heat-treated B-free carbon fibers.In other words, the fibers of the present invention are characterized byd₀₀₂ decreasing but Lc not changing drastically.

Also, as usual vapor growing fibers are heated, the peak at 1580 cm⁻¹ ofRaman spectrum increases and the peak at 1360 cm⁻¹ decreases, i.e., theR value decreases, finally to about 0.1 to about 0.2 as graphitizing,but in a boron treated product of the present invention, the R value was0.5 or more, about 0.7 to about 0.8.

Moreover, as the peak at 1580 cm⁻¹ became higher, the half maximum fullwidth narrowed to 20 to 40 cm⁻¹.

Along with decrease in d₀₀₂ and increase in the height of peak at 1580cm⁻¹, the electric conductivity increased to 0.01 Ω·cm or less,specifically 0.003 Ω·cm.

When shaped fibers having a high density obtained through press moldingor the like are heated, the fibers are partially sintered to therebyform a block. The heat-treated fibers as such cannot serve as a filleradded to an electrode or as an electron-emitting material. Thus, theshaped fiber must be disintegrated into a form suitable for a filler.

In order to do this, when the block is disintegrated, crushed, andclassified into a material suitable for a filler, non-fibrous matter isseparated simultaneously. Excessive crushing lowers performance of afiller, whereas insufficient crushing results in poor mixing with anelectrode material, to thereby lose the effect of a filler.

In order to produce a filler having a suitable shape, the block formedafter heat treatment is disintegrated to attain a dimension of 2 mm orless, and is further crushed by use of a crusher. Examples of adisintegrating apparatus which may be employed include an ice-crusherand a Rote-plax. Examples of a crusher which may be employed include apulverizer (impact type), a free crusher, and a Microjet. Non-fibrousmatter can be separated through classification, such as gas-flowclassification. The conditions for crushing and classification vary inaccordance with the type of the crusher and operational conditions. Inorder to fully realize a characteristic of a filler, the fiberspreferably have a length of 5-400 μm and an aspect ratio of 10 or more,more preferably 50 or more.

When the conditions are represented by bulk density after crushing andclassification, bulk density 0.001 g/cm³-0.2 g/cm³, preferably 0.005g/cm³-0.15 g/cm³, more preferably 0.01 g/cm³-0.1 g/cm³. When the bulkdensity is in excess of 0.2 g/cm³, the length of the fibers is as shortas 5 μm or less, depending on the fiber diameter, to thereby provide apoor filler effect; whereas when the bulk density is less than 0.001g/cm³, the length of the fibers is as long as more than 400 μm,depending on the fiber diameter, to thereby lower filling density of thefiller. The bulk density referred to herein represents tapping bulkdensity obtained from the volume and weight of fibers which are placedin a container and vibrated until the volume reaches an almost constantvalue.

The fine carbon fibers of the present invention are added to a batteryelectrode, to thereby enhance performance of the battery. The fibersenhance electric conductivity of an electrode plate of batteries such asa lithium battery, a lead secondary battery, a polymer battery, and adry battery, or the performance of a battery requiring an intercalatingproperty. In addition to an effect of enhancing conductivity of thesebatteries, the fine carbon fibers of the present invention, havingexcellent crystallinity and conductivity, are added to a lithium batteryto thereby enhance the charge-discharge capacity thereof, since thefibers exhibit a great intercalating property as a carbonaceous materialfor a negative electrode. Particularly, a carbon fiber in which d₀₀₂ is0.3420 nm or less and Lc is 40 nm or less provides the above effects toa considerable degree. Even in the case of boron-containing carbonfibers in which d₀₀₂ and Lc fall outside the above ranges, crystallinityand conductivity are more excellent than in the case of boron-free finecarbon fibers. Thus, the fibers are also adaptable to the above use.

The fine carbon fibers are preferably added to an electrode in an amountof 0.1-20 mass %. Amounts in excess of 20 mass % have an effect ofdecreasing the filling density of carbon in the electrode to therebylower the charge-discharge capacity of the produced battery; whereasamounts less than 0.1 mass % provide a poor effect of addition.

In order to produce a battery through the addition of the fine carbonfibers, a material for a negative electrode, the fibers, and a binderare sufficiently kneaded to thereby disperse the fibers as homogeneouslyas possible. Examples of a material for a negative electrode of alithium battery include graphite powder and meso-phase carbon microbeadsMCMB).

EXAMPLES

The present invention will next be described in more detail by way ofexamples, and effects of the carbon fibers of the present invention as afiller for an electrode will be elucidated.

Enhancement of the Crystallization Degree of Fine Carbon Fibers:

Example 1

Carbon fibers obtained through a known method (e.g., a method disclosedin Japanese Patent Application Laid-Open (kokai) No. 7-150419) in whichbenzene is thermally decomposed in the presence of an organic compoundcontaining a transition metal was used as a raw material, which wassubjected to a heat treatment at 1200° C. The resultant flocculatedfibers were disintegrated, to thereby produce carbon fibers having abulk density of 0.02 g/cm³ and a fiber length of 10-100 μm. Most of thefiber filaments had a fiber diameter of 0.5 μm or less (average fiberdiameter obtained through observation by use of an SEM image is 0.1 to0.2 μm). X-ray diffraction analysis revealed that the fibers had aninterlayer distance between crystal layers represented by d₀₀₂ of 0.3407nm and Lc of 5.6 nm.

The thus-produced fibers (2.88 kg) and B₄C powder (average particle sizeof 15 μm, 120 g) were sufficiently mixed by use of a Henschel mixer. Themixture was fed in a 50-L pipe-shaped graphite crucible and pressed byuse of a pressurizing plate made of graphite, to thereby adjust the bulkdensity to 0.075 g/cm³. The mixture was subjected to a heat treatment at2900° C. in an Acheson furnace while the mixture was pressed by thepressuring plate which simultaneously served as a cap in the crucible.Heating continued for 60 minutes after the temperature reached 2900° C.

After completion of heat treatment, the fibers were cooled, removed fromthe crucible, roughly disintegrated and then crushed by use of a bantammill. Non-fibrous matter was separated through gas-flow classification.

Although the fiber diameter of the resultant fibers did not change, thefibers had a length of 5-30 μm and a bulk density of 0.04 g/cm³. Theboron content and d₀₀₂ and Lc as determined by the X-ray diffractionmethod are shown in Table 1. The above heat treatment at 2900° C. wasrepeated except that B₄C was not added to carbon fibers. The results ofmeasurement of the above fibers are shown in Table 1 (the data indicatedby Comp. Ex. 1).

Example 2

Carbon fibers obtained in a similar manner as described in Example 1 wasdisintegrated and crushed, to thereby produce carbon fibers having abulk density of 0.05 g/cm³. Most of the fiber filaments had a fiberlength of 10-50 μm and a fiber diameter of average fiber diameter of0.06 μm obtained through observation by use of an SEM image. Thethus-produced fibers (150 g) and B₄C powder (average particle size of 10μm, 6 g) were sufficiently mixed by use of a Henschel mixer. The mixturewas fed in a cylindrical molding apparatus and pressed, to therebyobtain a column formed of carbon fiber having a diameter of 150 mm and abulk density of 0.087 g/cm³.

The shaped fibers were heated at 2800° C. under an argon flow for 60minutes in a graphite furnace having a heating medium made of graphite.

After completion of heat treatment, the fibers were removed from thefurnace, simply disintegrated to a dimension of 2 mm or less by use of amortar, crushed by use of a bantam mill, and subjected to gas-flowclassification. The obtained B-doped fibers had a bulk density of 0.046g/cm³. Most of the fiber filaments had a length of 5-20 μm.

The boron content and d₀₀₂ and Lc as determined by the X-ray diffractionmethod are shown in Table 1. The above heat treatment at 2800° C. wasrepeated except that B₄C was not added to the carbon fibers. The resultsof measurement of the above fibers are shown in Table 1 (the dataindicated by Comp. Ex. 2).

Example 3

Fine carbon fibers used as a raw material were obtained through the sameknown method disclosed in Japanese Patent Application Laid-Open (kokai)No. 7-150419 in which benzene is thermally decomposed in the presence ofan organic compound containing a transition metal. The carbon fibers,which were not subjected to a heat treatment, were disintegrated to abulk density of 0.01 g/cm³. Most of the fiber filaments had a fiberdiameter of 0.13 μm or less. The thus-produced fibers (200g) and B₄Cpowder (average particle size of 19 μm, 8 g) were sufficiently mixed byuse of a Henschel mixer. The mixture was fed in a shaping machine andpressed to form a cylinder having a diameter of 150 mm. The bulk densityafter the shaping was 0.07 g/cm³.

The shaped body was placed in a graphite furnace with a graphite heaterand was subjected to a heat treatment at 2800° C. for 60 minutes.

After the heating, the shaped body was removed, disintegrated to adiameter of 2 mm or less by use of a mortar and then crushed by use of abantam mill. Non-fibrous matter was separated through gas-flowclassification. The doped product had a bulk density of 0.03 g/cm³.

The boron content and d₀₀₂ and Lc as determined by the X-ray diffractionmethod are shown in Table 1. The above heat treatment at 2800° C. wasrepeated except that B₄C was not added to carbon fibers. The results ofmeasurement of the above fibers are shown in Table 1 (the data indicatedby Comp. Ex. 3).

TABLE 1 Boron content d₀₀₂ Lc (mass %) (nm) (nm) Example 1 1.03 0.338029.0 Comp. Ex. 1 — 0.3387 31.8 Example 2 1.10 0.3381 25.0 Comp. Ex. 2 —0.3398 26.9 Example 3 1.57 0.3382 31.1 Comp. Ex. 3 — 0.3401 21.7 Example4 1.02 0.3395 25.4 Comp. Ex. 4 — 0.3405 21.6 Example 5 0.93 0.3376 29.9Comp. Ex. 5 — 0.3383 30.5

Example 4

Carbon fibers obtained in the same manner as in Example 1 weredisintegrated and then crushed to have a bulk density of 0.02 g/cm³.Most of fiber filaments had a length of 10-50 μm and an average diameterof about 0.04 μm. 3000 g of the fibers was added with 120 g of B₄Chaving an average particle size of 15 μm and the mixture wassufficiently mixed by use of a Henschel mixer. The mixture (88 g) waspacked in a graphite crucible having an inner diameter of 100 mm and aninner 20 length of 150 mm. The bulk density of the packed mixture was0.08 g/cm³.

The crucible with a cover was placed in a carbon resistance furnace andheated in an argon flow at 2800° C. for 60 minutes.

After completion of the heat treatment, the shaped body was removed fromthe furnace and simply dissociated to a dimension of 2 mm or less by useof a mortar. It was then crushed in a bantam mill and classified by agas flow and the obtained boron-doped product had a bulk density of 0.04g/cm³. The boron analysis revealed that 1.02 % of boron was incorporatedin the crystals of the fibers. In Table 1, the found or measured valuesof d₀₀₂ and Lc are shown (d₀₀₂=0.3395 nm and Lc=25.4 nm). The values ofd₀₀₂ and Lc for carbon fiber which was treated in the same processexcept that B₄C was not added are shown as Comparative Example 4 inTable 1 (d₀₀₂=0.3405 nm and Lc=21.6 nm).

The powder resistance of the fibers was measured. The measuring methodwas developed by the inventors and is as follows.

As shown in FIG. 2, a measuring cell comprises a cell 4 having arequtangular of 10 mm×50 mm and a depth of 100 mm, a compressing rod 2and a receiver 3. A certain amount of powder is put in the cell and thecompressing rod 2 is pressed downward to compress the powder.

While the pressure and volume are measured, a current, 100 mA, is passedfrom an electrode 1 set in the direction normal to the pressingdirection and a voltage (E, V) for a 10 mm distance is read by use oftwo measuring terminals 6 extending from the receiver, and theresistance (R, Ωcm) is calculated from the following formula.R=E/100 (Ωcm)

The powder resistance depends on the density and the evaluation shouldbe made between values measured at the same density. In this case, theresistances measured at a powder density of 0.8 g/cm³ are compared.

The results are shown in Table 2.

The measured d₀₀₂ of the product was 0.3395 nm and Lc was 5.4 nm.

For reference, the resistance of a product produced in the same manner,but B₄C was not added, is shown as Comparative Example 9 in Table 2.

Example 5

Vapor grown carbon fibers produced in the same manner as in Example 1but having a diameter of 0.2 μm larger than that of Example 4 wereheat-treated at 1300° C. and dissociated to a bulk density of 0.05 g/cc.The fibers (150 g) and B₄C (6 g) having an average particle size of 10μm were put and mixed in a Hencshel mixer. The powder was shaped to around column with a size of 150φ×100 mm by use of a cylinder with aninner diameter and a length of 150φ×400 mm and a pressing device. Thebulk density of the body was 0.030 g/cm³, but it was pressed in thecylinder to have a compressed bulk density of 0.08 g/cm³.

The shaped body was placed in a graphite furnace with a graphite heaterand heated in an argon atmosphere with a temperature elevation rate of15° C./min. The heating temperature was 2800° C.

After the heat treatment, the fibers were removed from the furnace andlightly dissociated to a dimension of 4 mm or less by use of a mortar.They were crushed in a bantam mill and classified to have a bulk densityof 0.03 g/cm³. A boron analysis revealed that boron was incorporated inthe crystals of fibers in an amount of 0.93%.

The powder resistance of the obtained fiber is shown in Table 2.

The d₀₀₂ of Example 5 is simultaneously shown, which is lowered as thatof Example 4. Lc was 29.9 nm. For reference, d₀₀₂ and Lc of a productproduced in the same manner but B₄C was not added are shown asComparative Example 5 in Table 1.

TABLE 2 Powder resistance d₀₀₂ (Ω · cm) (nm) Com. Ex. 9 0.013 0.3388 (Bnot added) Fibers of 0.003 0.3395 Example 4 Fibers of 0.002 0.3376Example 5

Confirmation of Effect of Filler:

Example 6

In order to study if the powder resistance of a negative electrode isreally lowered when the fibers of the invention are added to theelectrode, the fibers of Example 4 were added to commercially availablegraphite particles (average particle size of 10 μm) and the relationshipbetween the added amount and the powder resistance was determined. Alsothe relationship between the added amount and the powder resistance whenthe fibers of Comparative Example 3, in which boron was not added, wereused, was determined.

The resistances for the cases when the fibers were not added and wasadded in an amount of 3%, 5% and 10% are shown in Table 3. The methodfor measuring the powder resistance was the same as in Example 4, butthe bulk density of the fibers was made to 1.5 g/cm³ since they arepreferably compared when the bulk density was set at the same density asthat of electrode.

Application of the above fibers to an electrode of a lithium batterywill next be described in an Example.

Example 7

An electrode was produced from the fibers alone (100%), and the effectof the fiber of the present invention was first investigated.

To each of the carbon fibers produced in Examples 1 to 3 and ComparativeExamples 1 to 3, PVDF (poly(vinylidene fluoride)) was added in an amountof 3 mass %. Each mixture was pressed onto nickel mesh, to therebyfabricate a working electrode (negative electrode). The batteryperformance was measured by use of Li metal as a counter electrode.LiPF₆ (1 mol) was dissolved in a mixture of ethylene carbonate (EC) anddiethyl carbonate (DEC) (volume ratio 1:1), to thereby prepare anelectrolyte for the battery. The current density during evaluation ofthe battery was 0.2 mA/g.

The measured discharge capacity of each battery is shown in Table 4.

Electrodes to which the carbon fibers were added will be described in anExample.

Example 8

Pitch coke was subjected to a heat treatment at 3000° C. to thereby formgraphite particles having an average particle size of 16 μm (G1), whichwas a carbonaceous material serving as a negative electrode material ofa battery. Graphite particles incorporated with boron during heattreatment (GB) and B-free graphite particles (G1) were used. GBcontained boron in an amount of 0.98 mass %.

PVDF (3 mass %) was added to each of GB, G1, and combinations of GB orG1 to which carbon fibers produced in Example 1 or Comparative Example 1were added in an amount of 5 mass %, to thereby prepare a slurry, whichwas then pressed onto nickel mesh to thereby produce an electrode plate.The electrolyte, counter electrode, and current density employed in thisExample are the same as those employed in the previous Example. Themeasurements of the discharge capacity of each battery are shown inTable 4. In Reference Examples 1 and 2, electrodes were constitutedexclusively by GB and G1, respectively. The calculated dischargecapacity in Table 4 was obtained through summation of 0.95-folddischarge capacity of G1 or GB and 0.05-fold discharge capacity ofcarbon fibers.

TABLE 3 Fiber addition Powder resistance (Ω · cm) (%) Fibers with no BFibers added with B 0 0.06 — 3 0.05 0.01 5 0.035 0.007 10  0.025 0.004

TABLE 4 Discharge Discharge capacity capacity Difference Electrode(mAh/g) (mAh/g) (found)- material (found) (calculated) (calculated)Example 7-(1) Fibers of 311 Ex. 1 Comp. Ex. 7-(1) Fibers of 275 Comp.Ex. 1 Example 7-(2) Fibers of 305 Ex. 2 Comp. Ex. 7-(2) Fibers of 272Comp. Ex. 2 Example 7-(3) Fibers of 308 Ex. 3 Comp. Ex. 7-(3) Fibers of269 Comp. Ex. 3 Ref. Ex. 1 GB 319 (B-added) Ref. Ex. 2 G1 280 Example8-(1) GB + 332 319 13 fibers of Ex. 1 (5 mass %) Comp. Ex. 8-(1) GB +321 317  4 fibers of Comp. Ex. 1 (5 mass %) Example 8-(2) G1 + 292 28210 fibers of Ex. 1 (5 mass %) Comp. Ex. 8-(2) G1 + 282 280  2 fibers ofComp. Ex. 1 (5 mass %)

As is clear from Table 4, when fine carbon fibers were added to acarbonaceous material serving as a negative electrode material, alithium battery having an electrode (negative electrode) formed of theabove electrode material exhibited a discharge capacity higher than thatof the value calculated from the discharge capacity of an electrodematerial and that of carbon fibers. Particularly, boron-incorporatedfine carbon fibers, to which boron was incorporated and treated at hightemperature to enhance crystallinity, exhibited the above effect. Inother words, addition of boron-containing fine carbon fibers to acarbonaceous material serving as a negative electrode materialdefinitely provides a synergistic effect, although the reason thereforhas not been elucidated. The effect is remarkably high whenhigh-crystallinity fine carbon fiber is added.

INDUSTRIAL APPLICABILITY

The present invention provides fine carbon fibers, having a higher levelof crystallinity which has not conventionally been realized, and finecarbon fibers containing boron. High-crystallinity imparts excellentelectric conductivity and thermal conductivity to carbon fibers. Thus,such carbon fibers are excellent material serving as fillers for resin,ceramics, or metal.

Particularly, the carbon fibers have a high dispersion efficiency due tosmall diameter thereof even in a small amount of addition is added as afiller for an electrode of a battery. Since the fine carbon fibers ofthe present invention can permit intercalation of significant amounts oflithium ions, the fibers are able to enhance the discharge capacity of abattery even with a small amount of addition.

1. A process for producing fine carbon fibers comprising: addingvapor-grown fine carbon fibers having a diameter of 1 μm or less toboron or a boron compound; compacting the mixture of fine carbon fibersand boron or a boron compound to obtain a solidified shaped mixture;relieving pressure after completion of shaping; regulating the bulkdensity of the mixture of fine carbon fibers and boron or a boroncompound during compacting such that the solidified mixture has a bulkdensity of 0.05 g/cm³ or more after pressure relief; and heat treatingsaid fine carbon fibers to a temperature of 2000° C. or higher whilesaid bulk density is maintained, to produce fine carbon fibers having aninterlayer distance d002 between carbon layers as determined by an X-raydiffraction method of 0.335 to 0.342 nm, and satisfyingd002<0.3448-0.0028 (blog φ) where φ represents the diameter the carbonfibers and the units of d002 are nm, and a thickness Lc of the crystalin the C axis direction of 40 nm or less.
 2. A process for producingfine carbon fibers according to claim 1, wherein the amount of the boronor boron compound is 0.1-10 mass % in terms of boron atoms with respectto the carbon fibers.
 3. A process for producing fine carbon fibersaccording to claim 2, wherein the fine carbon fibers to which the boronis mixed have a diameter of 0.01 to 1 m and an aspect ratio of 10 ormore.
 4. A process for producing fine carbon fibers according to claim3, wherein the fine carbon fibers prior to said heat treatment to whichthe boron or boron compound is to be added are a fired product ofvapor-grown carbon fibers fired after growth.
 5. A process for producingfine carbon fibers in according to claim 3, wherein the fine carbonfibers prior to said heat treatment to which the boron or boron compoundis to be added are unfired product of vapor-grown carbon fibers notfired after growth.